Probing the terahertz wave distribution inside a crystal in a terahertz wave parametric source

Terahertz (THz)-wave parametric sources are expected to be promising THz-wave sources, but there are few reports that experimentally evaluate how THz-waves develop during the generation process. In this study, we observed the THz-wave distribution inside a nonlinear optical crystal in detail by injecting a probe beam from the z-face of the crystal and upconverting THz-waves to near-infrared beam. By scanning the injection position of the probe beam, we measured the spatial distribution of THz-waves inside the crystal. As a result, we successfully captured experimentally the THz-wave amplification that depends on the seed beam intensity, and the shift of the generation position caused by pump depletion, both of which had previously been predicted by simulations. In addition, under high-intensity seed conditions, we observed a fringe pattern caused by interference of higher-order THz-waves, which is not considered in the simulations. These results not only deepen the physical understanding of the THz-wave parametric generation process but also support optimal source design and the development of more accurate numerical models.